1. Field of the Invention
The present invention relates to an iron-base article having a wear-resistant surface and a method of making such an iron-base article, and more particularly to an iron-base article having a remelted chill layer of high hardness in a region which requires resistance to abrasive wear and a method of making such an iron-base article.
2. Description of the Relevant Art
Iron-base articles such as camshafts or rocker arms have surfaces held in sliding contact with other members. Therefore, such surfaces are required be highly resistant to wear such as scuffing and pitting. As internal combustion engines are designed in recent years for higher power output, the cam lobe surfaces of cam shafts and the sliding-contact surfaces of rocker arms are subject to higher pressure, and they are required to be more and more resistant to wear.
One conventional way of casting such an iron-base article has been to set a chill in a mold and pour molten metal into the mold in contact with the chill, which rapidly cools and solidifies the casting to form a chilled layer of high hardness. With this conventional practice, however, the internal structure of the chilled layer is so coarse that the chilled layer does not have sufficient wear resistance. To eliminate this shortcoming, an effort has been made to mix a metal such as Cr, Mo, Ni, Cu or Mn for producing an iron-base article having a closer chilled structure of greater hardness. The effort has been unsuccessful because the addition of such a metal increases the hardness of an article region which is to be machined, with the result that the machinability of the article is lowered.
Another attempt has been to cast an iron-base article without forming a chilled structure, to employ a heater such as a TIG torch to remelt a region of the iron-base article which requires to be resistant to wear, and then to cooling the remelted region to produce a chilled layer. Where the iron-base article is a camshaft, this process is time-consuming and uneconomical since the camshaft must be remelted over its entire circumferential surface in order to lower the pressure imposed on the sliding-contact cam surface and also to prevent pitting from being generated. Furthermore, inasmuch as the side walls of cam lobes have bare cast surfaces, oxides on the cam lobe side walls tend to be trapped in the cam lobes when the cam lobes are remelted fully across their widths, thus producing gas holes or deforming the cam lobes.
Japanese Laid-Open Patent Publication No. 60 (1985)-234168 entitled "Remelted camshaft and method of making the same" discloses a method of remelting and hardening a camshaft of cast iron. The disclosed method comprises the steps of casting a camshaft while a chill is being disposed partly or fully over a cam profile to form a hardened chill layer, then applying high-density energy such as TIG welding arc or laser beam to the entire chilled surface laser of the sliding-contact cam surface thereby to heat and remelt the surface layer, and thereafter allowing the remelted surface layer to be cooled to form a hardened chill layer composed of cementite as a major phase fully over the sliding-contact cam surface. With this method, however, the hardness and wear resistance that can be attained are limited since the hardened chill layer formed by the chill is merely remelted by heating means such as a TIG torch and then cooled.
According to one conventional remelting and hardening process, as disclosed in Japanese Patent Publication No. 57(1982)-6494, a heater such as a plasma torch is positioned at a distance from a sliding-contact cam surface of a camshaft in confronting relation thereto, and the camshaft is rotated about its own axis while the plasma torch is reciprocally moved transversely across the sliding-contact cam surface, so that an arc produced by the plasma torch will be applied to the cam surface in a tortuous pattern. Where the cam lobe is heated and melted while the camshaft is being rotated about its own axis, the melted region is positioned higher than a previous region which has been melted immediately earlier. The melted region is therefore caused by gravity and the pressure of the arc from the plasma torch to flow over the previously melted region, thereby developing a deformed area on the cam profile. Such a localized deformation must be ground away after the remelting and hardening process. This is disadvantageous in that a large grinding allowance is required, a grinding crack may be developed, and the grinding process is complex and time-consuming.
The conventional remelting and hardening procedure requires a preheating step for rapidly melting a camshaft. It takes a considerable period of time for a plasma torch arc to travel from a point where preheating is started to a point where preheating is finished. As a consequence, it is difficult to maintain all cams to be remelted at the same temperature, and the camshaft cannot be remelted quickly.